pbmmask [-expand] [pbmfile]

       Reads  a portable bitmap as input.  Creates a corresponding mask bitmap
       and writes it out.

       The color to be interpreted as  "background"  is  determined  automati-
       cally.  Regardless of which color is background, the mask will be white
       where the background is and black where the figure is.

       This lets you do a masked paste like this, for  objects  with  a  black
           pbmmask obj > objmask
           pnmpaste < dest -and objmask <x> <y> | pnmpaste -or obj <x> <y>
       For  objects with a white background, you can either invert them or add
       a step:
           pbmmask obj > objmask
           pnminvert objmask | pnmpaste -and obj 0 0 > blackback
           pnmpaste < dest -and objmask <x> <y> | pnmpaste -or blackback <x> <y>
       Note that this three-step version works for objects  with  black  back-
       grounds too, if you don't care about the wasted time.

       You  can  also  use masks with graymaps and pixmaps, using the pnmarith
       tool.  For instance:
           ppmtopgm obj.ppm | pgmtopbm -threshold | pbmmask > objmask.pbm
           pnmarith -multiply dest.ppm objmask.pbm > t1.ppm
           pnminvert objmask.pbm | pnmarith -multiply obj.ppm - > t2.ppm
           pnmarith -add t1.ppm t2.ppm
       An interesting variation on this  is  to  pipe  the  mask  through  the
       pnmsmooth  script before using it.  This makes the boundary between the
       two images less sharp.

              Expands the mask by one pixel out from the image.  This is  use-
              ful  if  you  want  a little white border around your image.  (A
              better solution might be to turn the pbmlife tool into a general
              cellular automaton tool...)

       ppmcolormask(1),   pnmpaste(1),   pnminvert(1),   pbm(5),  pnmarith(1),

       Copyright (C) 1988 by Jef Poskanzer.
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